<canvas id="canvas" width="500" height="500" style="background:#000;"></canvas>

window.onload = function(){

  var canvas = document.getElementById('canvas'),  //获取canvas元素

      context = canvas.getContext('2d'),  //获取画图环境，指明为2d

      centerX = canvas.width/2,   //Canvas中心点x轴坐标

      centerY = canvas.height/2,  //Canvas中心点y轴坐标

      rad = Math.PI*2/100, //将360度分成100份，那么每一份就是rad度

      speed = 0.1; //加载的快慢就靠它了 

​

  //绘制5像素宽的运动外圈

  function blueCircle(n){

    context.save();

    context.strokeStyle = "#fff"; //设置描边样式

    context.lineWidth = 5; //设置线宽

    context.beginPath(); //路径开始

    context.arc(centerX, centerY, 100 , -Math.PI/2, -Math.PI/2 +n*rad, false); //用于绘制圆弧context.arc(x坐标，y坐标，半径，起始角度，终止角度，顺时针/逆时针)

    context.stroke(); //绘制

    context.closePath(); //路径结束

    context.restore();

  }

  //绘制白色外圈

  function whiteCircle(){

    context.save();

    context.beginPath();

    context.lineWidth = 2; //设置线宽

    context.strokeStyle = "red";

    context.arc(centerX, centerY, 100 , 0, Math.PI*2, false);

    context.stroke();

    context.closePath();

    context.restore();

  }  

  //百分比文字绘制

  function text(n){

    context.save(); //save和restore可以保证样式属性只运用于该段canvas元素

    context.strokeStyle = "#fff"; //设置描边样式

    context.font = "40px Arial"; //设置字体大小和字体

    //绘制字体，并且指定位置

    context.strokeText(n.toFixed(0)+"%", centerX-25, centerY+10);

    context.stroke(); //执行绘制

    context.restore();

  } 

  //动画循环

  (function drawFrame(){

    window.requestAnimationFrame(drawFrame);

    context.clearRect(0, 0, canvas.width, canvas.height);

    whiteCircle();

    text(speed);

    blueCircle(speed);

    if(speed > 100) speed = 0;

    speed += 0.1;

  }());

}